Tracking antenna systems dynamically follow changes in a direction of a received signal, and position a tracking antenna to align the signal with the peak level of the main beam of the tracking antenna. Such signal alignment results in coincidence of the signal and a portion of the main beam that provides maximum antenna gain and thus system sensitivity.
Various approaches to antenna tracking involve aligning the main beam peak of an antenna with the signal by using open commanding and relative power measurements. One example, referred to as “step track,” involves positioning an antenna in a nominal direction, and commanding the antenna in equal but opposite angular offsets and measuring received signal power at each offset position. If the received signal power levels are equal, the antenna is correctly aligned. If the received power levels are unequal, the difference in the power levels can be used to correct the antenna alignment. The process is repeated in the orthogonal plane. The step track technique is periodically repeated to validate correct antenna alignment and to follow any changes in the direction of the signal.
Other approaches to antenna tracking involve a closed loop technique referred to as “monopulse.” Two types of antenna patterns are used in such techniques. The first type of pattern has a maximum gain value that is coincident with the axis of the antenna, and is used for data reception. The second type of pattern has a null on the axis of the antenna and, to first order, has a linear variation with displacements from the axis and typically a phase difference between the data pattern and the tracking pattern that coincides with the azimuth angle of the signal direction. This behavior, the linear increase with deviation from axis and the phase difference, is used by an antenna control unit as an error signal, thereby permitting implementation of a closed loop tracking system that dynamically follows changes in the direction of the signal.
Unfortunately, these angle-tracking techniques depend upon initial antenna pointing (prior to initiation of antenna tracking) to align the signal within the main beam angular extent of the antenna. In some cases, however, such alignment is not assured.
One prior method for verifying this alignment involves using a smaller guard antenna together with the larger main antenna that is used for data reception. In practice, the smaller antenna is about 1/10 the diameter of the main antenna to obtain the required gain and pattern characteristics to envelope the sidelobes of the main antenna. The signal levels received by the main antenna and the guard antenna are then compared. If the signal level of the main antenna exceeds the signal level of the guard antenna, the antenna is aligned within the main beam where the main antenna gain is higher than the gain of the guard antenna. If the signal level of the main antenna is comparable or less than the signal level of the guard antenna, then the signal is aligned with sidelobes of the main antenna.
In addition to requiring a second antenna, another shortfall of this technique is that the boresight of the guard antenna needs to be maintained coincident with the main antenna. Moreover, the smaller guard antenna needs to be mechanically isolated from the main antenna to avoid deforming the main antenna and its patterns, and the mechanical balance of the assembly needs to be maintained. When the received signal level fluctuates, as commonly occurs with multipath at low elevation angles, the guard antenna requires a second tracking receiver so that the signal levels in the guard and main antennas can be simultaneously measured. Aside from the expense of an additional tracking receiver, the two receivers need to be reliably calibrated so that the same received power level results in the same indicated signal levels. This calibration is needed so that the signal level comparison can be used to reliably verify main beam alignment. Additionally, for large antennas that require a protective radome to avoid pointing errors caused by wind loading, a larger radome that envelops both the main and guard antennas is significantly more expensive than a radome for the main antenna alone.
Thus, it would be useful to be able to provide a main beam alignment verification alternative to the prior approaches. It would also be desirable to be able to provide a cost effective method for verifying the main beam alignment of received signals. It would also be useful to be able to provide a mechanism for verifying main beam alignment without imposing additional hardware capabilities, i.e., using existing antenna hardware.